Car Aerodynamics

Question 1

Slender body theory gives …

Answer 1

give the potential flow cross force per unit area at any position along a slender body

dCN/dx = qSing(2α) . dA/dx

Only where cross-sectional area changes

Question 2

List all three frictional coefficients of drag on slender body

Answer 2

Ctotal = Cp + Cf + Cb

Cp = pressure
Cf = friction
Cb = base pressure

Question 3

What happens to Cp, Cf and Cb on a slender body in subsonic flow

Answer 3

Cp = 0ish
Cf = largest contributor
Cb = significant and can be reduced using boat tail

Question 4

Why do vortices occur behind a car

Answer 4

Due to low pressure air on the top of the car, and high pressure air on the sides

Question 5

List 5 drag components on a car

Answer 5

1. Skin friction
2. Cooling drag - loss of colling-air momentum
3. Ventilation - mostly small unless window open
4. Form drag - largest contributor (air flow separation)
5. Lift-induced

Question 6

How to reduce drag on a car

Answer 6

• streamlining for form drag reduction
• reduce underside roughness
• fineness ratio
• reduce lift to decrease vortex drag
• exploit under body ground effects

Question 7

Where is Xcp situated on a car

Answer 7

For infront of Xcg, which cause car to yaw away from crosswind

Question 8

Where to position rear wing with respect to diffuser

Answer 8

The wing works better at producing down force when nearer the diffuser at the bottom of the car

Question 9

How does a diffuser work

Answer 9

Slopes rear of the car upwards
this allows air to escape quickly and so density reduction and therefore pressure reduction
the variation in pressure creates downforce

Question 10

What are the effects of skirting

Answer 10

Attempts to maintain low pressure under the body
creates stronger venturi effect
needs to be rigid with vertical freedom as suction changes with speed

Question 11

What happens to a wheels separation point when it rotates

Answer 11

The separation point moves forward